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The toxic chemical that sickened a Delaware family at a Virgin Islands resort last month had been used there other times over the past year, the Environmental Protection Agency said.

The EPA was notified on March 20 that the family was hospitalized after a fumigation days earlier at Sirenusa Resort in Cruz Bay, St. John. The agency found that the apartment below where the family stayed had been fumigated with methyl bromide.

An EPA spokeswoman said Thursday that the chemical was also used in other units in October 2014.

Methyl bromide, an odorless pesticide and neurotoxin, was banned for indoor residential use in 1984 by the EPA, which also holds authority over the U.S. Virgin Islands.

Initially, the EPA had said methyl bromide had been used in more than one apartment in March. But on Thursday, spokeswoman Mary Mears clarified that it was just one that month.

Inhaling the chemical can "be fatal or cause serious acute illness or delayed lung or nervous system injury," according to the EPA.

Stephen Esmond, his wife Theresa Devine, and their two teenage sons were hospitalized in Philadelphia after their exposure. Devine had been discharged as of Monday and Esmond is in stable condition — but the two boys are still in critical condition, a family spokesman told NBC News.

Methyl bromide was used in a first-floor apartment. Esmond and his family were staying on the second floor of that unit, Enck said.

Enck said the chemical was used by pest control company Terminix.

Sea Glass Vacations LLC, the company that manages many of the rental units at Sirenusa, said it was unaware Terminix allegedly used the chemical, and it has cancelled all of its contracts with Terminix.

The United States Department of Justice has also opened a criminal investigation into the case, according to a Securities and Exchange Commission report filed by ServiceMaster Global Holdings, Inc., the parent company of Terminix.

"We are sending our thoughts and prayers to the family impacted by this tragic incident, as well as those around them," Sea Glass Vacations said in a statement. "We remain committed to full cooperation with all local and federal authorities currently investigating this matter."

The company said only one unit was fumigated by Terminix in the recent case. Sea Glass Vacations said Terminix may have used the chemical when fumigating four other units in October. Terminix has been providing pest control at the property since late 2013. Terminix did not respond to a request for comment.

The EPA is investigating whether there have been any other reports of medical symptoms in the complex over the past year. The Agency for Toxic Substances Disease Registry is interviewing tourists who stayed in the apartments where methyl bromide was used and interviewing housekeeping staff, Enck said.

The Department of Justice could bring criminal charges forward if the EPA investigation finds evidence of wrongdoing.

"The first and top priority is protecting public health," Enck said. "Second is told responsible parties accountable for any violation of environmental laws."